The invention relates to an electric incandescent lamp comprising
a tubular glass lamp vessel sealed in a vacuum-tight manner, PA1 a filament of tungsten wire longitudinally arranged in the lamp vessel, PA1 current supply conductors extending through the wall of the lamp vessel to the filament, PA1 which filament has several helically wound light-emitting sections, which are interconnected by non-light-emitting sections integral with the light-emitting sections, a non-light-emitting section having at its two ends a respective helically wound portion, which portions are interconnected by the tungsten wire, PA1 an additional metal wire interconnecting the helically wound portions and a respective support bearing against the wall of the lamp vessel being present on said portions.
Such an incandescent lamp is known from DE GM 8 014 413.
An incandescent lamp of the kind mentioned can be used as a photocopying lamp. The variety of light-emitting and non-light-emitting sections has for its object to irradiate, to illuminate or to heat, uniformly throughout its width the article to be irradiated, an original or a copy made. With a lamp having a filament wound throughout its length with the same pitch this is not possible. At its edges the article would receive less radiation than halfway between the edges.
In a filament having light-emitting sections and non-light-emitting sections, the non-light-emitting sections could consist of one or more turns of very high pitch. These turns remain comparatively cold due to their very high pitch and consequently emit little radiation. If such a non-light-emitting section has a fairly great length, however, it nevertheless dissipates comparatively much energy and the efficiency of the lamp is low.
In order to obtain a lamp of comparatively high efficiency, numerous proposals have been made.
U.S. Pat. No. 3,416,024 discloses a lamp in which the filament is assembled by screwing light-emitting helically wound parts onto rod-shaped non-light-emitting parts. In the rod-shaped parts, the current density and hence the dissipation is low. The manufacture of this filament is time-consuming and expensive, however, because it requires much manual labour.
JP UM 151494/75 discloses a lamp having a filament, in which the non-light-emitting section is a straight wire portion, onto one end of which a metal wire is wound, which extends along said straight wire portion to its other end, is wound around it and is then spiralled out as a support. The non-light-emitting part is therefore bridged by an additional wire, as a result of which the electrical resistance is smaller.
A disadvantage of this known filament is that operations must be carried out on the slack assembly of light-emitting sections and straight intermediated portions to provide the bridging wire without deformations of the filament being obtained. The manufacture of this known filament is also time-consuming and expensive and requires much manual labour.
In the filament according to JP UM 77192/75, a non-light-emitting section consists of a straight portion of the winding wire, which has at both ends a number of turns. After the mandrel onto which the filament was wound had been removed, a straight wire bridging the non-light-emitting section is inserted into these turns. The wire is fixed on the turns by each time winding a support onto these turns.
The manufacture of this filament also requires very much manual labour and the manipulation of a body with vulnerable helically wound sections.
With respect to these known constructions, in which during the manufacture of the filament many constructive operations have to be carried out by hand on slack and vulnerable parts, the construction according to the aforementioned DE GM has the great advantage that all the constructive operations can be carried out mechanically on the filament while it is still strengthened by a winding mandrel.
According to the aforementioned DE GM, a tungsten wire is wound in accordance with the desired pattern around a molybdenum mandrel, after which supports are wound onto the filament. The filament then need no longer be subjected to any constructive operation. A protective layer is then formed on the portions of the filament which have to be non-light-emitting, after which the winding mandrel in the remaining portions is removed by dissolving it in an acid bath. Subsequently, the protective layer around the non-light-emitting portions is dissolved. The remainder of the winding mandrel is then present in these portions as a bridging wire.
This known filament has the great advantage that it can entirely be manufactured by machine. However, a disadvantage is that additional manufacturing steps deviating from normal production methods, i.e. providing and later removing the protective layers, have to be carried out.